OSPF Route Filtering and Summarization

OSPF Route filtering between areas

• Filtering prevents the creation of LSAs.
• LSDB’s have to be the same for all routers ifnot SPF logic will fail
• OSPF can filter the originiation of LSA between areas
• Type 3 LSAs are filtered prior to origination ABR
  • • With multiple ABRs filtering should be done on both.
    • ospf# area number prefix-list prefix name in|out
      • In: IOS filters routes comming in to that area
      • out: IOS filters routes comming out of that area

  ip prefix-list NAME seq 5 deny 10.10.10.0/24 le 32
  ip prefix-list NAME seq 10 permit 0.0.0.0/0 le 32
  
  R1#(config-router)#area 1 filter-list prefix NAME in
  R1#show ip ospf database summary self-originate

• Type 5 LSAs are filtered at ASBR

• Filtering with Area-Range
  • ospf# area number range <subnet><mask> not-advertise
    
  • Used for route summarization
  • not-advertise turns it into a filter
  • Doesn’t requite ACL or Prefix-Lists
  • Only works with Type 1 and Type 2 LSAs

R1#(config-router)#area 1 range 10.10.10.0 255.255.255.0 not-advertise

OSPF Route filtering into the Route Table

• Filter between OSPF database and Routing table.
• Router still have LSAs for the routes but after running SPF these routes are filtered using a distribute-list and don’t make it in the routing table.
• Can implemented on any OSPF router
• distribute-list prefix NAME in <interface>
• distribute-list ACL in <interface>

access-list 1 deny 10.10.10.0 0.0.0.255
access-list 1 permit any
R1#(config-router)#distribute-list 1 in

• extended acl to deny a route from a specific neighbor

access-list 101 deny ip host 1.1.1.2 10.10.10.0 0.0.0.255
access-list 101 permit any any 
R1#(config-router)#distribute-list 101 in

OSPF Route Summerization

• Only on ABR and ASBR
• Creates new LSA
• ASBR creates Type 5 LSA

ABR:
R1#(config-router)#area 1 range 1.1.0.0 255.255.252.0

ASBR:
R1#(config-router)#summary-address 1.1.0.0 255.255.252.0